To satisfy some standards or design requirements, a control amplifier is designed to be a linear-in-dB variable gain amplifier, that is, an output power and a control voltage of the control amplifier may have an exponential relation. However, the designs of the linear-in-dB variable gain amplifier may suffer some problems such as accuracy, process/temperature variations issue, and so forth. Therefore, it is important to provide a novel design to solve the above-mentioned problems.